Polyamides, such as nylon-6,6, require starting monomers of two kinds, a monomer having a pair of carboxylic acid functional reactive groups (diacid) and a monomer having a pair of amino functional reactive groups (diamine), and such polyamides are typically referred to as dimonomeric polyamides. The polyamide may further incorporate more than one diacid and more than one diamine and may incorporate a small amount, usually no more than 10%, of a third kind of starting material having a carboxylic acid functional group and an amino functional group or a functional precursor to such a compound.
In a conventional method of preparing such dimonomeric polyamides, the starting diacid and diamine components are mixed in stoichiometric proportions into an aqueous solution. The water is subsequently removed by evaporation, typically at elevated pressure in order to achieve a high enough boiling temperature to prevent the formation of solids. However, the post-evaporation pressure reduction step requires excessive heat to prevent the product from solidifying, and this heating is known to cause discoloration and chemical degradation of the product.
To avoid the use of water, alternative methods to produce polyamides comprise the supply of one or both components in liquid (molten) form. Typically, the polyamidation reactions are carried out in vertical multistage reactors, otherwise known as column reactors. The requisite diacid and diamine are flowed counter-currently through the reactor and the product polyamide collects at the lowest stages of the reactor, or column bottom. However, the high temperatures required to retain the component(s) in melt form can result in degradation, and a number of methods (see, for instance, U.S. Pat. No. 4,131,712, U.S. Pat. No. 4,433,146 and U.S. Pat. No. 4,438,257) have sought to reduce such degradation and overcome associated difficulties. U.S. Pat. No. 5,674,974 (incorporated by reference herein in its entirety) discloses the continuous production of polyamide in a vertical multistage reactor with counter-currently flowing dicarboxylic acid and diamine streams, which improved earlier processes by reducing energy consumption, reducing capital cost of equipment and reducing environmental emissions, as well as improving product quality. In vertical multistage reactors such as that disclosed in U.S. Pat. No. 5,674,974 the diacid feed stream typically consists of a mixture of diacid and diamine in which there is an excess of diacid. Such a diacid feed-stream does not require supra-atmospheric pressures in order to solvate in the moisture produced by the polyamidation reaction, and thus the reactors are operated at atmospheric pressure. The flow of diamine fed into the reactor is typically controlled to maintain a stoichiometric balance of diacid and diamine.
In all such methods comprising the supply of component(s) in liquid (molten) form, it is a requirement that the molten material accumulating at the bottom of the reactor must be homogeneous and sufficiently mixed, in order for the reaction to proceed efficiently. Agitation is essential to column operation in order to homogenize the lower three stages and avoid gel build-up in stagnant zones, which can also cause degradation and the formation of coloured impurities. Gel build-up occurs because, without direct means for controlling the chemical equilibria in the melt, the temperature rises due to the heat emitted by the polyamidation reaction which in turn causes evaporation of water produced by the polyamidation reaction thus causing a rise in viscosity and gel build-up. For example, as the temperature reaches ca. 250° C. mid-way down the column reactor the moisture in the liquid melt falls below ca. 0.5 wt % and approaches 0.2 wt %. The melt is thus starved of moisture, thereby promoting viscosity rise. All such conventional processes therefore require mechanical agitation in order to attain sufficient mixing. However, there are several disadvantages associated with the use of mechanical agitation, including reactor complexity and complexity in process scale-up. Agitators used in vertical multistage polyamidation reactors are complicated and expensive to design and manufacture as they require adequate mechanical strength to sufficiently agitate molten polyamide, but minimal surface area and roughness in order to limit the extent to which their surface provides nuclei for gelation. Larger reactors require commensurately larger mechanical agitators. However, as the size of the agitator is increased to cope with the increased size of the reactor, it becomes increasingly difficult to transmit the torque generated by the agitator across the diameter of the column. Moreover, as the size of the mechanical agitator increases, its mechanical strength must also increase, which leads to difficulties in the design, fabrication and reliability of the component, as well as increased capital expenditure. The effective limit on the size of the mechanical agitator in turn limits the size of the polyamidation reactor, and hence the production output. In addition, processes and apparatus using conventional mechanical agitation are sensitive to perturbations of material in the reactor, and can suffer from poor reliability.
It is an objection of the present invention to overcome one or more of these problems.
As used herein, the term “counter-currently flowing” has the meaning conventional in the art, namely the direction of the current of one flow stream is opposite to the direction current of another flow stream in the reactor.
As used herein, the term “salt” is used in a general sense to encompass the precursors to polyamidation whether in a fully ionized state, an oligomeric state, or in any combination thereof.
As used herein, the term “weir” has its meaning conventional in the art, namely a barrier which impedes the flow of liquid phase reaction fluid. The weir causes liquid phase reaction fluid to pool behind it, while allowing liquid phase reaction fluid to flow steadily over the top of it once a sufficient volume of reaction fluid has built up behind it. Thus, a weir preferably comprises a surface which is perpendicular or substantially perpendicular to the direction of flow of the liquid phase reaction fluid at the point of contact of the weir with the reaction fluid, although any appropriate angle greater than 0° (preferably at least 30°, preferably at least 60°, preferably at least 85°) to the direction of flow of the reaction fluid may be used.